1. Field of the Invention
The present invention relates to a multiplex voltage measurement apparatus, and specifically to a multiplex voltage measurement apparatus for measuring a voltage of each of serially connected N voltage sources.
2. Description of the Related Art
A high-power electric source of several hundred voltages for an electric vehicle is formed by a number of secondary battery cells, such as nickel-hydrogen storage cells, which are serially connected to each other. Each of the serially connected battery cells should be monitored for its capacity for the purpose of charge/discharge control. In particular, a battery formed by 240 serially connected cells produces a total voltage of 288 V. In such a battery, it is physically difficult to monitor each cell. In Japanese Laid-Open Publication No. 8-140204, for example, the voltage is measured for each of 24 modules each including 10 cells.
In an electric vehicle, high-voltage systems are electrically insulated from a chassis in order to avoid hazardous conditions. On the other hand, since a processor for charge/discharge control uses a potential of the chassis as a reference potential, the voltage of a battery should be insulatively measured. In the battery disclosed in Japanese Laid-Open Publication No. 8-140204, an insulation circuit unit including an operational amplifier, an AD converter, a photocoupler, a power supply, etc., is provided for each module. The structure of such a battery is enormously complicated.
As means of insulatively measuring the output voltage of a sensor or the like, a flying capacitor is known. FIG. 3 shows a structure of a multiplex voltage measurement apparatus 400. In this example, the number of voltage sources (N) is 5.
Serially-connected voltage sources V1-V5 are connected to a capacitor 3 through voltage detection terminals T1-T6, and through a first multiplexer 1 formed by switches S1, S3, and S5 and a second multiplexer 2 formed by switches S2, S4, and S6. The capacitor 3 is connected to a voltage measurement circuit 6 through a first sample switch 4 formed by switches 4a and 4b and a polarity correction circuit 5.
FIG. 4 is a timing chart for opening/closure of the respective switches S1-S6, and 4a and 4b. An operation of the multiplex voltage measurement apparatus 400 is now described with reference to FIG. 4 in conjunction with FIG. 3.
Prior to measuring the voltages of the voltage sources V1-V5, the switches S1-S6, and 4a and 4b are all opened (OFF). During period P1, first of all. the switches S1 and S2 are closed (ON), whereby the voltage of the voltage source V1 is applied to the capacitor 3, and a charge is stored in the capacitor 3. After being kept closed (ON) for a predetermined time period, the switches S1 and S2 are turned off. Then, after a predetermined time has elapsed since the switches S1 and S2 were turned off, the first sample switch 4 (switches 4a and 4b) is turned on, whereby the charged voltage in the capacitor 3, i.e., the voltage of the voltage source V1, is transferred to the polarity correction circuit 5 and the voltage measurement circuit 6.
As a matter of course, a driving circuit of each switch and a contact point of the switch are kept separated. The first multiplexer 1 is not closed while the first sample switch 4 is closed, and the second multiplexer 2 is not closed while the first sample switch 4 is closed. Therefore, the voltage of the voltage source V1 is insulatively measured, i.e., when the voltage of the voltage source V1 is measured, the voltage source V1 and the capacitor 3 are insulated.
During period P2, the switches S2 and S3 and the switches 4a and 4b are similarly turned on and off, and. during period P3, the switches S3 and S4 and the switches 4a and 4b are similarly turned on and off. In this way, as shown in FIG. 4, the multiplex voltage measurement apparatus 400 operates in a multiplex manner.
It should be noted in FIG. 3 that the voltage value of an odd-numbered voltage source is inverted by the polarity correction circuit 5 so as to have the same polarity as that of the voltage value of an even-numbered voltage source before it is transmitted to the voltage measurement circuit 6. An example of the polarity correction circuit 5 is shown in FIG. 5.
The polarity correction circuit 5 shown in FIG. 5 is a well-known absolute-value circuit. The polarity correction circuit 5 changes the polarity of a voltage to be input to an AD converter of the voltage measurement circuit 6 such that all of the voltages to be input to the AD converter have the same polarity. The polarity correction circuit 5 is effective for a unipolar voltage source such as a battery. It should be noted that the polarity correction circuit 5 is not limited to such an analog circuit, but may be a digital circuit which is independent of a polarity output bit of a bipolar input-type AD converter.
Referring again to FIG. 3, the multiplex voltage measurement apparatus 400 further includes an electric leakage detection circuit 7. The electric leakage detection circuit 7 includes a second capacitor 10, amplifiers 11 and 12, a signal generation circuit 13, and a level comparison circuit 14. The electric leakage detection circuit 7 compares output levels of the amplifier 12 and the signal generation circuit 13, thereby measuring the insulation impedance between the N voltage sources and the chassis.
However, in the conventional multiplex voltage measurement apparatus 400 having the above-described structure, if a line between the voltage detection terminal T6 and the switch S6 which is connected to the electric leakage detection circuit 7 is disconnected at position 15 (shown by xe2x80x9cxxe2x80x9d in FIG. 6), a loop L (represented by a line with arrows in FIG. 6) is formed. As a result, an electric charge is stored in the first capacitor 3, which should not be generated when the switches S1-S6 are open, and accordingly, a voltage is generated therein. Therefore, breakage of the line connected to the voltage detection terminal T6, e.g., disconnection at position.15, cannot be detected.
According to one aspect of the present invention, a multiplex voltage measurement apparatus includes a multiplex voltage measurement section for measuring a voltage of each of N serially connected voltage sources and an electric leakage detection circuit for measuring an insulation impedance between the N voltage sources and a chassis, the multiplex voltage measurement section including, (N+1) voltage detection terminals connected to the N voltage sources, a first capacitor having a first terminal and a second terminal, a first multiplexer for selectively connecting any of odd-numbered voltage detection terminals among the (N+1) voltage detection terminals to the first terminal of the first capacitor, a second multiplexer for selectively connecting any of even-numbered voltage detection terminals among the (N+1) voltage detection terminals to the second terminal of the first capacitor, a voltage measurement circuit for measuring the voltage between the first terminal and the second terminal of the first capacitor, a first sample switch for connecting the first terminal and the second terminal of the first capacitor to the voltage measurement circuit, and a polarity correction circuit for changing the polarity of voltages of the N voltage sources such that the voltages of the odd-numbered voltage sources among the N voltage sources and the voltages of the even-numbered voltage sources among the N voltage sources have the same polarity, wherein the multiplex voltage measurement section measures the voltages of the N voltage sources by repeating a process in which the first and second multiplexers select one of the N voltage sources, and the first and second multiplexers are opened while the first sample switch is closed, and the electric leakage detection circuit including a second capacitor connected to one end of the N voltage sources, a signal generator for supplying a signal through the second capacitor to the one end of the N voltage sources, a level comparison circuit for comparing a signal level of the signal generator and a signal level of the second capacitor, and a second sample switch provided between the signal generator and the one end of the N voltage sources.
In one embodiment of the present invention, the second sample switch is provided between the second capacitor and the one end of each of the N voltage sources.
In another embodiment of the present invention, the second sample switch is provided between the second capacitor and the signal generator.
In still another embodiment of the present invention, the second sample switch is opened only when the multiplex voltage measurement circuit measures one of the N voltage sources which is located at the one end of the N voltage sources.
According to the present invention, in order to eliminate the above-described problem in the conventional voltage measurement apparatus, a second sample switch is provided for opening/closing a line between the signal generator 13 of the electric leakage detection circuit 7 and one end of the N voltage sources in the conventional multiplex voltage measurement apparatus 400 shown in FIG. 6.
The second sample switch is opened only when measuring one of the N voltage sources which is located at the one end of the N voltage sources. Otherwise, the second sample switch is closed.
With such an arrangement, even when the line between the voltage detection terminal T6 and the switch S6 is disconnected, the loop L described above is not formed, and an electric charge is not stored in the first capacitor 3. Therefore, the disconnection of the line between the voltage detection terminal T6 and the switch S6 can be easily detected. It should be noted that the voltage sources except for the first and last voltage sources of the serially connected N voltage sources are not connected to the electric leakage detection circuit, and thus, the loop L is not formed with respect to these voltage sources.
Furthermore, the second sample switch may be provided between the second capacitor 10 and one end of the N voltage sources, or between the second capacitor 10 and the signal generator 13. In either case, the same effects can be obtained.
Thus, the invention described herein makes possible the advantage of providing a multiplex voltage measurement apparatus which can detect breakage of a line which is provided between a voltage detection terminal and a switch corresponding thereto and which is connected to an electric leakage detection circuit.
This and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.